Various kinds of rolling bearing units for vehicle wheel comprising outer and inner rings rotatably combined with each other through rolling members are used to rotatably support a vehicle wheel to a suspension. The rolling bearing unit for rotatably supporting a driven wheel, that is also a steering wheel, such as a front wheel of the FF vehicle or of the 4 WD vehicle is combined with a constant velocity joint to transmit smoothly the rotation of the driving shaft to the vehicle wheel, keeping the constant velocity property, regardless of the steering angle of the vehicle wheel.
JP Utility Model Publication KOKAI NO. S61-113103 discloses a conventional rolling bearing unit for vehicle wheel combined with a constant velocity joint, which can be relatively compact and lightweight.
FIG. 2 shows the conventional structure disclosed in this publication. An outer ring or race 1 is supported by the suspension so as not to rotate when incorporated in the vehicle, and formed with a first mount, flange 2 in a radially outward flange shape on its outer peripheral surface to be supported by the suspension and with first and second outer ring raceways 3, 4 in double rows on its inner peripheral surface. A hub 5 and first and second inner rings 6, 7 are provided on the radially inside of the outer ring 1. On the outer peripheral surface of the hub 5, at a portion closer to the axially outer end, a flange 8 is provided to support a vehicle wheel to the hub 5, and integrally formed with the hub 5.
The terms "axially outer" and "axially outside" mean the widthwise outside when installed in the vehicle, left in FIGS. 1 and 2 while the terms "axially outer" and "axially inside" mean the widthwise inside when installed in the vehicle, right in FIGS. 1 and 2, in the present specification.
A plurality (usually 4 to 6) of studs 9 are provided circumferentially with a uniform interval with the base ends thereof axially press-fitted into the flange 8. When mounting the vehicle wheel to the suspension, the vehicle wheel is fixedly supported by the flange 8 with the studs 9.
The hub 5 has an axially inner end portion for a housing portion 11 to be the outer ring of the constant velocity joint 10.
The hub 5 has a middle portion onto which first and second inner rings 6, 7 are fitted with first and second inner ring raceways 12, 13 in double rows formed on their outer peripheral surfaces. A plurality of rolling members 14 are provided between the first and second outer ring raceways 3, 4 and the first and second inner ring raceways 12, 13 to rotatably support the hub 5 and the first and second inner rings 6, 7 inside the outer ring 1.
The annular space where the rolling members 14 are provided has opposite opening portions, where the gaps between the opening portions on the opposite ends of the outer ring 1 and the outer peripheral surface of the first inner ring 5 at the axially outer end and the outer peripheral surface of the second inner ring 6 at the axially inner end are covered by seat rings 15, respectively.
The outer peripheral surface of the hub 5 is formed generally circumferentially with an anchoring groove 16 at a portion in the middle portion on the side of the axially inner end, and a stop ring 17 having a semicircular shape is anchored in the anchoring groove 16.
The flange 8 has a step portion 18 at the base end thereof on the axial inside, and the first and second inner rings 6, 7 are supported from the opposite axial sides by the stop ring 17 and the step portion 18. In this state, the stop ring 17 prevents the first and second Miner rings 6, 7 from moving toward the axial inside of the hub 5.
A boot 19 made from a resilient member such as rubber, synthetic resin, and in a bellows shape is provided such that die axially outer end portion of the boot 19 is fitted onto on the outer peripheral surface at the axially inner end of the housing portion 11. The axially outer end portion of the boot 19 is formed with a cylindrical portion which is fitted onto the axially inner end portion of the housing portion 11 and retained by the retaining band 20.
Formed on the outer peripheral surface at the axially inner end portion of the housing portion 11 are engagement grooves 21 which are generally circumferentially engaged with the inner peripheral surface at the axially inner end portion of the boot 19.
The other end of the boot 19 is tightly connected to the outer peripheral surface (not shown) at the middle portion of the drive shaft 22 rotatably driven by the engine through the transmission.
The boot 19 isolates the inner space of the housing portion 11 from outside and prevents the grease in this inner space from leaking out to the outside, and the foreign matter such as rain water, dust from entering this inner space.
When incorporating the front wheel or driven wheel to the vehicle using the rolling bearing unit constructed as mentioned above, the outer ring 1 is fixedly supported by the suspension by way of the mount flange 2, and the front wheel is fixedly supported by the hub 5 by way of the flange 8.
The drive shaft 22 is rotated by the engine through the transmission and the tip end of the drive shaft 22 is engaged in a spline joint with the inside of the inner member or ring 23 of the constant velocity joint 10.
When the vehicle is moving, the rotation of the inner ring 23 is transmitted to the hub 5 through the balls 24 to rotate the driven wheel, e.g. front wheel.
On the other hand, DE 19547981 (U.S. Ser. No. 365,354 filed in 1994 with claiming priority) discloses an example of rollers used for the rolling members on both of the axial inside and outside.
There is a problem in the conventional structure as shown in FIG. 2 as follows; specifically in the case of the conventional structure, the rolling bearings in double rows provided inside the outer ring 1 supported by the suspension to rotatably support the hub 5 with, the vehicle wheel fixed thereto, have the common structures and dimensions in the rolling member row portions. Therefore, the rolling member row portions have the same basic dynamic load rating. In the case of the double row rolling bearing unit, if the same load is added to the rolling member row portions, the rolling member row portions have the substantially same life. However, in the generally used automobiles, the rolling member row portions are subjected to different loads, such that the load applied to the axially inner rolling member row portion (right row in FIG. 2) is larger than the load applied to the axially outer rolling member row portion (left row in FIG. 2).
In addition, the rolling member row portion on the axial inside is placed close to the constant velocity joint, and subjected to more severe use conditions, e.g. heat generation and load from the constant velocity joint, than the rolling member row portion on the axial outside.
Accordingly, the life of the axially inner rolling member row portion is shorter than the life of the axially outer rolling member row portion, which is not desirable on the design of the rolling bearing unit. For example, the construction parts are not effectively utilized.
If the diameter of the balls for the rolling members on the axial inside is made larger, or if the number of the balls for the rolling members on the axially inside is increased, to substantially equalize the lives of the both rolling member row portions, the diameter of the bearing section would be become larger, which is not desirable for design.
The structure of FIG. 1 of DE 19547981A1 where the rollers are used for the rolling members, would be more costly than the case where the balls are used for the rolling members.